Turn a TableGen FastISelEmitter warning into an error.

Tablegen emitted a warning when the fast isel emitter created dead
code by emitting a pattern that has no predicate before a pattern
that has one.

This should be an error but was originally only a warning because the X86
backend had a buggy definition that unintentionally caused this to be hit
(PR21575). That has been fixed a while ago (r222094), so it's safe to
upgrade the warning to an error.

llvm-svn: 293534